Help!!! I smell gas coming thru A/C!!!

I just had the head gasket replaced in my 98 Civ LX at the beginning of the month, and ever since the work was done, every time I turn on the A/C for an extended period of time, I smell gas coming through the vents. And it's not just a whiff, these fumes are overpowering! The original work was done at the dealership, and we've taken it back twice. The first time, they steam cleaned the engine, and we didn't smell anything for a couple days, and then it came back, just as strong. We took it back yesterday, and the service manager drove it home himself yesterday, and claims he checked all the seals, and he can't smell or find anything wrong. He's saying that there must have been gas caught in the Catalytic Converter, and it just blew out. Personally, I'm not so convinced. Any thoughts out there?