If you have CH4, the molecule has no dipole moment because it is symmetrical. If a H is pulled off and a Cl is substituted, you have a dipole because the Cl is much more electronegative the the hydrogen atom, the electrons are pulled closer to the Cl than they are to the C, so the Cl has a slight negative charge on it and the C has a slight positive charge on it; thus, the dipole is created.
