The difference between the two boiling points is 119.5 oC. Propane is the larger molecule due to its relative molecular mass being higher then Methane and due to this the strength of the force is higher. Propane has more electrons and nuclei therefore the intermolecular van der waal attractions are stronger then Methane which has less electrons making propane a molecule with a stronger boiling point and stronger intermolecular forces.The difference between these compounds are the boiling points which Vary.
Butane being Van der waals has the lowest boiling at -140 (oC) then with the second highest with hydrogen bonding is propan-2-ol which has hydrogen bonding and then with the highest boiling point is Propanone which has Dipole-Dipole forces. Van der waal forces for Butane are week and their strength is not very high making the boiling point relatively low. Hydrogen bonds for Propan-2-ol make the intermolecular forces of the compound stronger with more electrons making the boiling point high but not as high as dipole dipole forces. Dipole-Dipole forces present in Propanone are very strong making the boiling point very high.