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Nalis struck in the 37th minute to settle a scrappy game in favour of Argyle who have not reached as high as the 13th place they now occupy since the opening week of the season.
The midfielder's goal came against the run of play as Brighton, struggling for their Championship life, threatened to upset the home side's five-match unbeaten home run.
A long throw from Tony Capaldi on the left-hand side was not dealt with by the Brighton defence and the ball eventually broke for Nalis, who lobbed it back into an unguarded net from around 12 yards.
It was the first goal for ex-Leicester man Nalis since his transfer-window move to Argyle following his release by Sheffield United.
Pilgrims forward Nick Chadwick should have twice settled the game in the final few minutes of the first half.
The former Everton striker broke free on the left and drove into the penalty area. His shot from a tight angle was easily saved by Brighton goalkeeper Wayne Henderson, with strike partner Vincent Pericard waiting for a pull-back.
Chadwick then snatched at a chance after Mat Doumbe had cut the ball back.
After a second half devoid of chances for either side, Brighton were left to reflect on two early first-half opportunities that went begging.
Argyle goalkeeper Romain Larrieu had to dive low to his right in the fifth minute to keep out a diving header from young Brighton striker Joe Gatting.
Gatting, nephew of former England cricket captain Mike, should also have done better than screw a shot across the face of goal a few minutes later.
Nalis struck in the 37th minute to settle a scrappy game in favour of Argyle who have not reached as high as the 13th place they now occupy since the opening week of the season.
The midfielder's goal came against the run of play as Brighton, struggling for their Championship life, threatened to upset the home side's five-match unbeaten home run.
A long throw from Tony Capaldi on the left-hand side was not dealt with by the Brighton defence and the ball eventually broke for Nalis, who lobbed it back into an unguarded net from around 12 yards.
It was the first goal for ex-Leicester man Nalis since his transfer-window move to Argyle following his release by Sheffield United.
Pilgrims forward Nick Chadwick should have twice settled the game in the final few minutes of the first half.
The former Everton striker broke free on the left and drove into the penalty area. His shot from a tight angle was easily saved by Brighton goalkeeper Wayne Henderson, with strike partner Vincent Pericard waiting for a pull-back.
Chadwick then snatched at a chance after Mat Doumbe had cut the ball back.
After a second half devoid of chances for either side, Brighton were left to reflect on two early first-half opportunities that went begging.
Argyle goalkeeper Romain Larrieu had to dive low to his right in the fifth minute to keep out a diving header from young Brighton striker Joe Gatting.
Gatting, nephew of former England cricket captain Mike, should also have done better than screw a shot across the face of goal a few minutes later.