[clevermartin]

It was suggested that both the melting point and boiling point increase in the order:
RCH₂F < RCH₂Cl < RCH₂Br < RCH₂I

Isn't RCH₂F molecule being the most polar, and should have the highest meltin point?

[clevermartin]

Can someone help me please?

[Padfoot]

I would say that the increasing number of electrons leads to greater dispersion forces.  This must outweigh the point you made in your post (about stronger dipole-dipole attractions).  Hopefully someone can verify this though...

[LQ43]

boiling points:
CH3-F -78.4oc
CH3-Cl -24.2oC
CH3-Br 3.6oC
CH3-I 42.4oC

yes, it would seem that the dipersion forces outweigh the usually stronger dipole-dipole attractions.

the balance between electronegativity differences leading to dipole-dipole attractions and the strength of dipersion forces seems not readily predictable when H-bonding is not involved.