Hellickson (10-2) gave up a run on four hits and a walk with five strikeouts to win his sixth straight decision. The 20-year-old right-hander hasn't given up more than two earned runs in any of his last eight starts and is fourth in the South Atlantic League with a 2.42 ERA.
Neal Frontz picked up his 15th save, despite surrendering a run on three hits with a strikeout in two innings.
Stewart went 2-for-4 and slugged a solo shot, his 17th, in the sixth for the Catfish (25-16), who have won three straight. The 23-year-old right fielder has four homers and nine RBIs in his last 10 games.
Brett Grandstrand belted an RBI double in the second and Seth Dhaenens drove in two runs for Columbus.
Stewart and Ryan Royster are tied for the team lead in home runs, with 17.
The win closes the Catfish to 1-1/2 games behind division-leading Asheville, who lost to Delmarva, and keeps them 1/2 game behind Charleston, who beat Greensboro in 11 innings.