The only way Sheheen could possibly be running for reelection in 2018 would be if he defeats Mulvaney for the 5th district in 2016.
Which is unlikely since he has a career as is in a non-term-limited State Senate. If I were him, I would challenge the weaker Rice in SC-7 considering they both represent Chesterfield (though he would have to run in 2016 to have a chance at winning, so he would probably have to give up his seat to do that) or maybe try for Governor again when the seat is open? Those are probably his best and only options.
Sheheen was born, raised, and has lived all his life in Camden, which is in Kershaw County, which is in the 5th. He'd have to carpetbag to face Rice, so if for some improbable reason Sheheen were to go after a Congressional seat it needs to be Mulvaney's. Mulvaney isn't all that much stronger than Rice and certainly not enough to face the downside of being called a carpetbagger. I certainly wasn't suggesting such a run as a desirable option for him, just that it was the only option that could even remotely give him a chance to be running for reelection in 2018 for any office.