The scope of this work is to find an optimal operating strategy for a fixed bed reactor system with a slowly deactivating catalyst. Catalyst deactivation occurs practically in all fixed bed reactors, and optimal operation of reactor systems undergoing catalyst deactivation is an important economic issue. The process studied is Lurgi's methanol synthesis, but the method also applies to other fixed bed reactor systems. An economic objective has been maximized with respect to coolant temperature and recycle ratio over a fixed time horizon by control vector parameterization and a sequential optimization method; An optimal operation strategy resulting in 1,800,000 USD increase in profit over four years compared to a reference case is found.