Argyle central defender Paul Wotton smashed in a 25th-minute penalty following Jon Parkin's far-post foul on home skipper Conor Hourihane following a corner.
Striker Jason Banton doubled the Pilgrims' lead with a 20-yard strike for his fifth goal in nine games since joining on loan from Crystal Palace.
Argyle keeper Jake Cole made a brilliant reflex save to keep out Junior Brown's goal-bound chest-down in first half stoppage time but he was beaten by David Ball with 15 minutes left.
Sheridan said: "It was a massive win, a great result to come away with three points from a game against a good team after getting off to a good start and going 2-0 up.
"The only disappointing thing I would say is that when you are winning a game 2-0, we shouldn't have made it as hard as we did.
"We should be controlling the game when we are 2-0 up and that is something that I have got to try and get into them.
"I don't think they know how to win. I don't want to be harsh when I say it but that's why I think they have been struggling if they've ever been in a winning position. They have just got to know how to win a game.
"Even when we win a game, we shouldn't be hanging on in there at the end.
"I thought we looked comfortable at 2-0 at half time, Jake's made a great save just before half time.
"But I thought we played well, a good really solid performance in the first half to get 2-0 up so then just see the game out, do the basics.
"Decisions we make then end up causing us problems and we end up conceding free kicks, corner and we had to defend more than we should have been."