The boiling point of halocarbons can be explained on the basis of strength of intermolecular forces of attraction. Haloalkanes show dipole dipole interactions. C-F bond is more polar and the polarity of the C-halogen bond decreases as we move down the group. But as the distance between C-X bond increases from C-F to C-I, the overall dipole moment decreases. Although haloalkanes show dipole dipole interactions, dispersion forces dominate the dipole interactions. The molecular size of halogen atom increases from fluorine to iodine as we move down the halogen group. With an increase in atomic size and molecular mass, the strength of london dispersion forces increases. So, the boiling point of haloalkanes increases from HF to HI.
Order of increasing boiling points: Fluoromethane>Chloromethane>bromomethane>iodomethane