The purpose of this work is to find an optimal operating strategy for a reactor system with a slowly deactivating catalyst. The process studied is Lurgi＇s methanol synthesis, but the methods also applies to other fixed bed reactor systems. The catalyst deactivates irreversibly and must be replaced after some years. The catalyst life time is considered to be known. The problem is formulated as a dynamic optimization problem with time-varying coolant temperature as the free variable. A sequential method is used to solve the dynamic optimization problem.