An olefin polymerization process comprises gas-phase polymerization of at least one olefin monomer in more than one polymerization zones in one or more polymerization reactors using a high activity catalyst injected in the front end of the reactor to give solid polymer particles. According to the process of the invention, different hydrogen to olefin ratios are controlled and applied to the reactor leading to the production of very different molecular weights and therefore broadening the molecular weight distribution of the polymer produced.