Can somebody tell me how can methadone help heroin addicts?

In many ways: Opiate addicts mostly use very short-acting drugs like heroin or oxycodone, which they have to dose 4-6 times a day, and their bodies and brains (and minds) are constantly cycling between high-normal-low, with very destructive effects on neurotransmitters in the brain. Methadone (or suboxone) is long acting, can be taken once daily, and stops this vicious cycle. For some its shortterm, others long.

Methadone: Methadone is an opiate just like heroin is however it does not provide the same euphoria as a heroin does. You can get high on it however if you use enough. It does block emotional respond. So it helps by not getting you high with a normal dose you do not go into withdrawals. You do not have as much craving. It keeps you from the criminal lifestyle needed to support a heroin at habit.