A late penalty was all that separated the Revs and D.C. United.
The Revs didn't start well, with Scott Caldwell deflecting home an own goal to give Untied a lead just 11 minutes in.
Diego Fagundez pulled the Revs level early in the second half, slipping behind the defense and slotting home the equalizer despite calls for an offside from the visiting team.
One minute after the Fagundez goal, Nguyen had a chance to take the lead from the penalty spot but was saved by United goalkeeper Bill Hamid.
Nguyen got his revenge in the 83rd when he earned a second trip to the spot for New England. This time the midfielder made no mistake and gave his team a vital win in its chase for the playoffs.
The win leaves the Revs just one point above the Fire, with New England sitting fifth in the East.