If methane had a high melting point, then it would also have a high boiling point.
But the premise of your question is incorrect.
Methane has a low melting point of -182 °C.
That means that the attractive forces among the methane molecules must be very small.
It takes very little energy to separate the molecules from each other in the solid and get them into the liquid phase.
It also takes very little energy to separate the molecules from each other in the liquid phase and get them into the gaseous state.
The boiling point of methane is still a relatively cool -161.5 °C.