Process for the gas-phase polymerization of olefins in a fluidized bed reactor, by a) passing a fluidizing gas containing an olefin monomer through a fluidized bed of polymer particles in the presence of a polymerization catalyst, b) withdrawing a first gaseous stream containing solid particles from the reactor, c) passing the first gaseous stream to a gas/solids separator, separating solid particles, and forming a second gaseous stream containing residual solid particles, d) passing a portion of the second gaseous stream to a heat exchanger(s) to remove heat and e) recycling a portion of the cooled stream from (d) as the fluidizing gas in (a). The fouling rate of the heat exchanger (s) is such that i) the increase in pressure drop across the heat exchanger is equivalent to less than 5%/year, and/or ii) the decrease in heat transfer of the heat exchanger is equivalent to less than 5%/year.