In this paper, a new multivariable proportional-integral (PI) tuning strategy is developed and the advantage of the new design is illustrated in simulation on an internal combustion engine model. The multivariable control design technique developed here has disturbance rejection as its main objective rather than set-point tracking, which is the focus of most of the multivariable PI tuning techniques so far in the literature. The benefit of the new design is that it does not try to minimize cross couplings in the multivariable plant; instead it uses the cross couplings to achieve better disturbance rejection. The application of the control design method is in multivariable speed and air/fuel ratio control of a lean-burn natural gas engine to achieve smooth and effective idle speed regulation. When applied to a simulation model of the engine, the new PI tuning strategy effectively reduces speed undershoot during the application of a transient torque load during idle.