How do you find heatran in Pokemon platinum?

To find or catch Heatran, you need to have already finished the Stark Mountain quest which is available after you defeated the Sinnoh Elite Four and got the National Pokedex.

Where Charon and the Team Galactic Grunts were captured by Looker and the International Police, Heatran will be there.

Heatran will be level 50 and have he following moves:

Metal Sound

Crunch

Scary Face

Lava Plume