Sean Monahan scored on a power play midway through the third period, lifting the Calgary Flames over the Dallas Stars 2-1 on Tuesday night.Johnny Gaudreau passed to Monahan from behind the net into the slot, and he slid a wrist shot past goalie Kari Lehtonen through a narrow opening at the bottom left corner of the net. Stars coach Lindy Ruff challenged the goal, saying that Calgary was offside, and the decision was overturned.Dallas had an apparent goal nullified with 4:35 to play when the referees determined that the puck had been kicked into the net. Lehtonen had 18.Dallas’ Curtis McKenzie opened the scoring with an unassisted goal in the first period.Each team had an apparent goal disallowed.The teams had few good chances in the first period. The Stars are 1-3-1 in the last five.Monahan assisted on Gaudreau’s goal that tied it at 1 in the second period. He beat Johnson with a wrist shot.Dallas outshot Calgary 9-7 in the first, but the Stars didn’t have another shot on goal until Julius Honka’s slap shot at 7:28 of the second period.With the teams skating 4-on-4, Lehtonen stopped a breakaway by Gaudreau. Gaudreau has two goals and two assists in two games since recovering from a broken finger.Flames goalie Chad Johnson made 29 saves. Gaudreau passed across ice to Chiasson, who passed back to Gaudreau on the left for a snap shot with Lehtonen pulled out of position.The Flames appeared to take a lead at 15:23 of the second, when Mark Giordano drilled a slap shot into the net. The goalie stretched out his right skate to deflect the shot wide left.Gaudreau and the Flames got even at 12:19. Monahan sent a pass to Gaudreau, and he skated in on Lehtonen 2-on-1 with Alex Chiasson. McKenzie scored at 13:15 after he took the puck away from TJ Brodie in the neutral zone and skated down low in the right faceoff circle. The Flames won their fourth straight and are 6-1-1 in the last eight games.