Each gene in this molecule gives the instructions for making

 Iodine (I) and chlorine (Cl) are both in group 7 of the periodic table. Since electronegativity decreases as you go down a group,  Cl will have a greater attraction for the pair of shared electrons that forms the bond between it and I. We can have a dipole forming because of this thus; dipole-dipole interactions can occur between identical ICl molecules.

As the bromine atoms in Br2 are identical, shared electrons that form the bond between the atoms are not pushed towards one or the other, because both have an equal attraction. Therefore, no dipole can occur and these electrostatic interactions between the molecules cannot be seen with Br2. Thus, this is why you would expect ICl to have a higher boiling point