Right before the building goes down (in the film and in real life), Heath walks out of it in a nurse's outfit and wig, pushes a trigger button, and it beeps but nothing happens. He tries several more times, and small propane explosions occur behind him. It's not the building going down, they rigged separate small explosions.
Finally, he gets pissed off and goes into a school bus, and RIGHT as he closes the door, the demolition starts, and they got this all in one shot. Wally described how the camera tracked with Heath, etc etc, but I wasn't exactly paying attention as I was writing all of this down.
But yeah, he said, "It wasn't JUST a first take for the building, it was a first take with Heath too!" They had to reinforce the glass in the bus and everything, to make sure Heath wouldn't get hurt. Wally also gushed about Heath's performance, and his ability to do this scene (expressions and all) in one take, cause there were no second chances.