Answer is:
1) in molecule of methane (CH₄) all the atoms around the central element (carbon) are equivalent (four hydrogens) and it has a net dipole moment of 0 (vectors of dipole moments cancel each other, dipole moment is zero).
2) in molecule of chloromethane (CH₃Cl) there are different elements (one chlorine and three hydrogen atoms) surrounding the central atom (carbon) and it has higher net dipole moment.