EJ, I agree with calgal. I'm far from a doctor, but my limited understanding of how methadone works is that it coats your nerve receptors, and so, if you go off of it, it will take a while before other pain killers will work, and it will also take up to a few days before you feel the pain/terrible physical withdrawal from it. As for pain med.s if you're already on methadone, I have no idea, but an anesthesiologist would. My advice would be, despite your fears, definitely be honest with them about your concerns and your past. Hopefully they will figure something out that will work for you personally. Best wishes.
EJ, you need to sit and have a talk with the anesthesia person who is going to work with you while the surgery is being done. Only someone with experience in this issue will be able to adequately answer your questions.