A typical natural gas plant currently in operation is studied. In the present work, a previous model has been extended to rigorously represent the compression and separation areas. Optimal operating conditions have been determined for varying feed flowrates. Different turboexpansion processes have also been analyzed through a MINLP strategy. The optimization program is an implementation of the Outer Approximation algorithm that is interfaced to the simulation program. The structural and parameter optimization of the entire plant has been performed to determine the optimal turboexpansion economic flowsheet.