A method includes receiving, at a master MPC controller from a slave MPC controller, proxy limit values indicating to what extent the slave controller is able to change multiple manipulated variables in multiple directions within a variable space without violating process variable constraints of the slave controller. The variable space includes a first feasibility region defined by the process variable constraints. The method also includes estimating a second feasibility region associated with the slave controller. At least part of the second feasibility region resides within the first feasibility region. The method further includes performing plantwide optimization at the master controller. A solution generated during the plantwide optimization includes a combination of manipulated variable values within the second feasibility region. Estimating the second feasibility region includes identifying edges of the second feasibility region based on the proxy limit values. At least some of the edges meet at locatio
