A nonlinear model-based control scheme is presented which extends nonlinear internal model control to systems with constrained inputs and measured disturbances. A method is proposed for a direct synthesis nonlinear controller which solves an instantaneous optimization to minimize the performance loss associated with enforcing the actuator constraints. Within this framework, the nonlinear controller can be represented as an optimal linear controller with an auxiliary feedback loop which cancels the effects of nonlinear dynamics and measured disturbances. The methods developed within the paper are applied to three simulation examples : a constrained linear example with measured disturbances, an isothermal reactor with Van de Vusse kinetics, and an industrial styrene polymerization reactor.