Heimer can (and always has been able to) single-handedly dominate a lane. His turrets protect him from harassment, ganks, and minions, and are nearly impossible to take out until level 10+. I've seen several matches where Heimer is 18 with the rest of the players around 13. You know there's a problem when I fear a nest of Heimer's turrets more than an inhibitor tower. A simple fix would be to limit the amount of turrets he can place, and/or make them despawn if he gets a certain distance away from them.
His rockets are a problem as well. Any passive ability that does huge damage to multiple champions from an extreme range is going to cause a problem. There needs to be some element of skill involved in using his rockets, and the range needs a major nerf.
His grenade is fine. It's pretty powerful compared to other skillshots but its slow as well. The only fix is to make it not damage towers, like every other skillshot in the game (besides Twitch).
He's not brutally overpowered, but he is extremely frustrating to play against.
the only way heimer to be at lvl 18 and the others at 13 is if hes soloing on some lane, and any other champ soloing a lane will level up faster than others... also you are completely forgeting how champs are built, heimer doesn't start at level one with 6 turrets, overpowered multi target rockets and crazy grenades, a good heimer will choose only one path at a time, so if you are afraid of his turrets then there's nothing to be afraid of his rockets and nades, and so on...
his rockets only do damage up to 3 targets when maxed out, his granades a stupid slow, if you are hit by one then its your fault and thats the reason they do both stun and blind, because they will one stun you if you get it in the center of the nade explosion, if not, they will just blind you, what a problem huh!?