Thanks so much, haha. That is beyond awesome.
In reference to its counters, here is a list of Pokemon that can adequately counter this combination, in my opinion, and some Pokemon that you may think can, but in reality, cannot.
Counters:
Infernape: A strong counter to many variations of GrowthTran, Infernape scares this combination. With Close Combat, it can easily dispatch of Heatran. However, keep in mind that Flare Blitz is a physical attack. It will not take down Tangrowth in one hit -- in fact, there is even a chance, depending on the held item and nature of the opponent, that you can use Morning Sun to completely cure you of the damage, effectively Power Point-Stalling the Infernape into hurting itself with each blow. However, to a Choice Band or Life Orb Infernape, Tangrowth will barely survive and has no business really staying in the battle. Pack a ghost or fighting resist, and you should be set. Also, if you decide to stick Choice Scarf on Heatran, it can absorb the fire attack and kill Infernape with its boosted fire attack. Infernape is only a counter to the Life Orb-variation or Choice Specs.
Charizard: Resisting every single attack in the combined set, Charizard is a definite threat. Switch out immediately. Charizard, truthfully, is the only absolute counter to this set, since Infernape fails against the Choice Scarf variation. While the Choice Scarf variation out-speeds Charizard, its attacks probably will not kill it before it kills you. However, there is still a chance that with the boosted speed, Heatran can kill it with a STAB, Flash Fire, Sunny Day Fire Blast.
Blissey: Blissey, of course, causes major problems, especially to the Choice Scarf set. However, the Life Orb and Choice Specs sets can take this pink blob down in two hits, provided you are not crippled by Thunder Wave.
Aerodactyl: While Ice Fang from Aerodactyl is merely a 4HKO against Tangrowth, it will not be offering up any Heatran blessings with the absence of fire moves and the damaging Earthquake. Just make sure you dont get caught in the misprediction, since it has such a diverse move-pool.
Gyarados: Gyarados is *not* a counter! Ice Fang is a 3HKO on Tangrowth, and Tangrowth will also 3HKO it back with Grass Knot. Unlike Gyarados, Tangrowth has Morning Sun. Also, Heatran will out-speed Standard Bulky Gyarados with any item and can OHKO with Choice Specs and all of the set-up. Just be careful how much you risk if Gyarados is packing Earthquake. Waterfall is no big deal in Sunny Day.