To balance a chemical equation, one must keep track of the atoms on each side of the equation as you change the coefficients in front of each component.

Initially   CH3Cl  + O2  ---->  CO2 + H2O  + Cl2   has

Reactants                                  Products

C    one                                            C one

Cl   one                                            Cl two

H    three                                          H  two

O   two                                             O  two +  one  =  three

If we balance the Chlorine first by multiplying the CH3CL by 2 we get

2CH3Cl  +  O2  ----->  CO2 + H2O + Cl2

Reactants                                Product

C  two                                          one

Cl  two                                          two

H   six                                           two

O  two                                           two + one = three

We then balance the Carbon by multiplying the CO2 by 2 and the Hydrogen by multiplying H2O by 3

2CH3Cl  +  O2  ---->  2CO2  +  3 H2O  +  Cl2

Reactants                                Products

C  two                                       two

Cl  two                                      two

H   six                                      six

O  2                                         four + three = 7

We can now balance the oxygen by multiplying the O2 by 7/2

2CH3Cl  + 7/2 O2  ---->  2CO2  + 3H2O  + Cl2

However, we cannot have a fractional coefficient, so we eliminate the fraction by multiplying each component by a factor of 2

4CH3Cl  +  7O2  ---->  4CO2  + 6H2O  +  2Cl2

Reactants                    Products

C  4                             4

H  12                           12

Cl  4                              4

O  14                            8 + 6 = 14