The present invention relates to a process for the gas-phase polymerisation of olefins in a fluidised bed reactor, which process comprises: a) passing a fluidising gas comprising one or more olefin monomers through a fluidised bed of polymer particles in the presence of a polymerisation catalyst, b) withdrawing a first gaseous stream comprising solid particles from the top of the reactor, c) passing the first gaseous stream to a gas/solids separator, separating solid particles therefrom, and forming a second gaseous stream comprising residual solid particles, d) passing at least a portion of the second gaseous stream to one or more heat exchangers to remove the heat of reaction, and e) recycling at least a portion of the cooled stream from step (d) as the fluidising gas in step (a), characterised in that the rate of fouling of the one or more heat exchangers is such that i) the increase in pressure drop across the heat exchangers is equivalent to less than 5% per year, and/or ii) the decrease in heat transfer of the heat exchangers is equivalent to less than 5% per year.