Alternate between Bulldoze and Low Kick until he's down!

Since you're faster, use Roar to drown out any Attracts/Taunts.

EDIT: Also, Darkly, can you double-check your damage calc for the last Low Kick I used? 'Cause I'm suddenly realizing that 14 HP seems a low, considering Heatran is Steel and almost weighs as much as Snorlax...