Brett Connolly, Nicklas Backstrom, John Carlson and Tom Wilson scored, and Braden Holtby made 28 saves for the Capitals. The five goals are the most allowed by Holtby since he gave up six against the Chicago Blackhawks on Feb. 17, a span of 30 games.
Game 2 of the best-of-7 series is here Wednesday.
"I think as a whole we can play a lot better, which is exciting to me," Washington coach Barry Trotz said. "I know we have another level in our game. It's got to be our whole roster."
Reaves has scored in back-to-back games, including the series-clinching goal in Game 5 of the Western Conference Final against the Winnipeg Jets, after he did not score in his first five games in the Stanley Cup Playoffs and final 23 in the regular season (21 with Vegas after being acquired Feb. 23 in a trade from the Pittsburgh Penguins).
Nosek has scored three goals in three games after not scoring in his first 10 postseason games.
"It's great when you see those guys get rewarded," Vegas coach Gerard Gallant said. "They're a big part of our group. It's nice to see those guys chip in with some goals. They got three huge goals for us tonight in the third period."
There were four lead changes, with the Golden Knights giving up leads of 1-0 in the first period and 3-2 in the second, and the Capitals blowing leads of 2-1 in the first and 4-3 in the third.
"A lot of times you feel like the game is going in one direction, and then they score and we score," Vegas defenseman Nate Schmidt said. "A lot of emotion. You've just got to make sure you keep it in check. It was a pretty emotional game for everybody.
"The fans got their money's worth tonight."