How do you catch Heatran in Pokemon X/Y?

Question

Two people were talking about it in Jaune Plaza - I think you can catch it somehow. Can you?
It has been driving me crazy! I think you need to mimic a Pokemon cry in order for it to show.

Answer 1

They are big, fat liars. LIIIEEEEEZ

Unfortunately you can only receive it by migrating it from another game. Sorry. D:

Hope I helped. :)
Source: http://www.serebii.net/pokedex-xy/485.shtml

Answer 2

There is no verified method of catching Heatran in X and Y.

However, from my speculation, the "Lava Dome Pokemon," might be Volcanion, a possible event Pokemon revealed from GTS exploits, since I see no logic in holding back an already existing Pokemon.

Either way, this "Lava Dome Pokemon" is nowhere to be found in X & Y... At least for now.