I just did a research on Narcotics (Opiates) and Methadone is highly addictive, but it's the best thing to do if you are withdrawing from Heroin. The withdrawals from Methadone will be less intense, so what it's meant to do is progressively get you out of the Opiates. I would do as your Dr. says, if he's a trustworthy Dr.
Do you have any references?
Hope it helps.