Stadler had fallen of the pace after sharing the third-round lead with Sim. Stadler closed with a 4-over 76 to fall into a tie for 16th at minus-7.
Meanwhile, Sim was going for his first tour title. He parred the 17th and 18th holes Monday to force a playoff with Ken Duke, who closed with a course- record, 7-under 65. Sim finished with a 71 to force the playoff at 12-under-par 276.
At the par-5 18th, which served as the playoff hole, both players found the green with their third shots. Duke had 9 feet for birdie and Sim had 7 feet.
Duke was unable to convert his birdie effort, but tapped in for par. Sim stepped up and drained his birdie putt for the win.
A nearly three-hour weather delay on Sunday forced the first Monday finish on the Nationwide Tour since 2001.
This is the first time in the 17-year history of the Nationwide Tour that regulation has seeped over to Monday. There have been three Monday finishes for playoffs with the last coming in 2001.
Sim and Stadler shared the third-round lead, but Sim took a two-shot lead into the suspension of play. Stadler made a birdie putt after the three-hour stoppage, but three bogeys in a row knocked Stadler out of contention.
Duke flew up the leaderboard thanks to four birdies and an eagle on the front nine. He tallied birdies at 11 and 14, then converted a 10-foot birdie putt at the last to get in at minus-12.
Sim only had birdies at both front-nine par-5s, the third and sixth. He bogeyed 10, but parred his next six before the horn sounded to stop play because of darkness.
Jarrod Lyle shot a 1-under 71 and finished at minus-10.
Roland Thatcher (67), Cameron Beckman (67), Garrett Willis (68), Tom Johnson (69), Chad Collins (69), Chris Tidland (69) and Craig Lile (70) finished at 9-under-par 279 in a tie for fourth.