The method of inequalities is applied to the of design a robust PI controller for a multi-stage flash (MSF) desalination plant. The method takes into account the given variations in process gain, time delay and time constant (K-p, tau(d) and tau) of the transfer function models. The performance of the closed loop system is evaluated for different operating points (i.e., on different transfer function models). PI controllers are also designed using an internal model controller (IMC) and by Ziegler-Nichols (Z-N) methods based on the nominal transfer function model. The performances of these controllers are compared. The controller designed by the method of inequalities gives the best results.