Process for the gas-phase (co-)polymerization of olefins in a fluidized bed reactor

A process for the gas-phase copolymerization of ethylene with a comonomer, comprising copolymerizing ethylene with a comonomer selected from the group consisting of but-1-lene, pent-1-ene, 1, 4-methylpent-1-lene and oct-1-ene in a fluidized bed reactor using a Ziegler-Natta catalyst containing a transition metal, said process being performed in the presence of an at least one additive selected from the group consisting of ethyl chloride, propyl chloride, butyl chloride, pentyl chloride, hexyl chloride and heptyl chloride and said additive being added to the reactor an amount of from 0.2 to 40 moles per mole of transition metal in the catalyst in the reactor.