Adiabatic tubular reactors are used in many industrial processes. The conventional control scheme for these reactors is to maintain the inlet temperature to the reactor. However, the optimum operation of the entire process (reaction and separation sections) often requires that the reactor be run at the highest possible temperature, which occurs under steady-state conditions at the reactor exit if the reactions are exothermic. The process studied in this paper has the exothermic, irreversible, gas-phase reaction A + B --> C occurring in an adiabatic tubular reactor. A gas recycle returns unconverted reactants from the separation section. Four alternative plantwide control structures for achieving reactor exit temperature control are explored. The reactor exit temperature controller changes different manipulated variables in three of the four control schemes: (1) CS1, the set point of the reactor inlet temperature controller is changed; (2) CS2, the recycle flow rate is changed; and(3) CS3, the flow rate of one of the reactant fresh feeds is changed. The fourth control scheme, CS4,uses an "on-demand" structure. Looking at the dynamics of the reactor in isolation would lead one to select CS2 because CS1 has a very large deadtime (due to the dynamics of the reactor) and CS3 has a very small gain. Dynamic simulations demonstrate that in the plantwide environment, with the reactor and separation operating together, the CS3 structure gives effective control and offers an a attractive alternative in those cases where manipulation of recycle flow rate is undesirable because of compressor limitations. The on-demand CS4 structure is the best for handling feed composition disturbances.