We use the ammonia reactor to develop further a method that minimizes the entropy production rate of a reactor with one chemical reaction. The method presented here determines operating conditions that are compatible with minimum lost exergy, i.e., the minimum total entropy production rate in the system. The entropy produced in the system by the chemical reaction and the heat transfer from the reaction mixture to the cooling medium is expressed by irreversible thermodynamics. The total entropy production rate is minimized subject to a fixed ammonia production. The mole and energy balances of the system must also be fulfilled. The results give inlet conditions compatible with the minimum entropy production rate. The results suggest that the reactor can be shortened or more high-quality heat can be taken out from the process. The nature of the solution, or the optimal temperature and concentration profiles in the reactor and the chemical and thermal driving forces, is presented and discussed.