I am about to have knee surgery and I am on methidone and they are putting me under on anesthesia and I do not want to tell my doctor is it fatal?

Consultation. You will need to consult with your surgeon and the anesthesiologist. The Methadone can depress the central nervous system and put you at risk. You will also need to talk about pain and pain medication during your recovery, as that will interact with your Methadone dose as well.
Not fatal. you should tell them as it may affect your anesthestic requirements.
No, it is. not fatal, but knee surgeries can be very painful and it is better to know about Methadone to regulate your narcotic analgesia requirements. If you stay in the hospital you will not receive your Methadone unless you let them know.
Truth is best. I'm thinking that Methadone is a step in the right direction for you. Also, if you care enought to fix the knee, care enough to get good medical treatment. They need to know your Rx history to do their best during and after surgery. Here's a similar query - you may have read it. I agree with the advice to tell the truth. https://answers.yahoo.com/question/index?qid=20110106140952AAF8WGg.