It was the first time this season that the Union (5-3-2, 17 points) scored two goals in a game, and the result put the Union back in first place in the Eastern Conference.
"It was a good step forward," Union manager Peter Nowak said. "We're not going to take the foot off the pedal right now."
All three goals came in a 12-minute span late in the second half. The first came in the 63d minute, when rookie midfielder Michael Farfan got the Union on the scoreboard with a long-range strike of his own.
After Roger Torres was fouled deep into the Fire's half of the field on the left flank, Kyle Nakazawa played the free kick back across midfield. Farfan ran forward and shot the ball from almost 30 yards out. With a wall of defenders blocking his view, Conway was frozen, and could only watch as the ball rolled to his right and into the net.
Farfan did not have long to celebrate, as Chicago (1-4-5, 8 points) tied the game in the 67th minute. Gonzalo Segares played a cross from the left side that went almost the full width of the field to Dominic Oduro, who beat Farfan off the dribble and shot past Faryd Mondragon from a very acute angle.
After scoring the decisive goal, Ruiz was taken out in the 81st minute for defender Gabriel Farfan. The Guatemalan forward received a loud ovation, with a few "Chooch!" chants sprinkled in from the Sons of Ben supporters club.
Nowak had to make his first substitution of the night in the game's opening moments. Midfield Amobi Okugo suffered a high left ankle sprain after slipping as he tried to kick a loose ball upfield.
Although the game was fairly open, it was also chippy at times. Referee Hilario Grajeda gave out six yellow cards, four to Chicago and two to the Union.
After a scoreless first half, Sebastien Le Toux nearly got the Union on the board in the 58th minute, trapping a ball in front of Conway and drawing the Chicago goalkeeper off the goal line. Le Toux was able to spin the ball around Conway, but it hit the post and stayed out of the net.
With just over 30 minutes left to go in the game, Nowak went for the win, bringing on Roger Torres for Jordan Harvey.
As has been the case many times this season, the introduction of Torres - who has fast become a fan favorite at PPL Park - proved a major factor in the game. The diminutive 19-year-old Colombian drew the free kick that livened up a gridlocked game, and the floodgates opened after that for a thoroughly entertaining final half hour.