1. A method comprising: obtaining a planning model for an industrial facility at a master model predictive control (MPC) controller; sending at least one optimization call from the master MPC controller to one or more slave MPC controllers; receiving at least one proxy limit value from the one or more slave MPC controllers at the master MPC controller in response to the at least one optimization call, wherein the at least one proxy limit value identifies to what extent one or more process variables controlled by the one or more slave MPC controllers are adjustable without violating any process variable constraints; and performing plantwide optimization at the master MPC controller using the planning model and the at least one proxy limit value, wherein the at least one proxy limit value allows the master MPC controller to honor the process variable constraints of the one or more slave MPC controllers during the plantwide optimization.