The operating conditions for a heat-integrated ethylene quench column are optimized and diagnosed for controllability and resiliency. Since a bank of seawater coolers largely meets the column＇s cooling duty requirements, the heat-exchanger by-pass flow fractions were optimized as a function of seawater temperature, using a specially adapted genetic algorithm. The controllability and resiliency of the quench column are studied and two alternative MIMO control configurations are investigated. Of particular interest was the impact of the nominal operating position of the bypass on disturbance rejection capacity. It was found that the resiliency of the column could be significantly enhanced by repositioning the nominal operating point of the main bypass valve and at the same time, the overall heat recovery in the seawater coolers could be enhanced, leading to significant savings in operating costs. (C) 2000 IFAC.