Process for removing heat from at least two reactors forming at least part of a polymerization reactor system for a multimodal polymerization in which the reactors are linked in series and all produce at least one component of the same polymer. The process includes cooling a stream of fluid in one or more heat exchangers and passing part of it to the cooling system of a first reactor and part of it to the cooling system of a second reactor, so as to remove the heat from said reactors, and passing back through the heat exchangers a return flow having a portion of the combined exit flows of the cooling fluids used to remove the heat from each of the reactors. The heat exchangers provide at least 90% of the cooling requirement for the reactors, and a portion of the combined exit flows from the reactors bypasses at least one of the heat exchangers and is passed directly to the cooling system of one or more of the reactors. The cooling circuit is powered by no more than one pump at any one time.