Confidence in the accuracy of a simulation model and by implication the thermodynamic data model(s) used, particularly for separation equipment such as a deethyleniser, requires validation against actual plant operating data. A methodology has been developed that quantifies plant-model mismatch with a Calculator module appended to a PROM (Simulation Sciences, Inc.) Column module to facilitate an assessment of the relative accuracy of alternative simulation model formulations. The methodology is based on a manual stepwise optimization procedure using a preferred formulation of the simulation model with the key adjustable modeling parameter defined as the effective average equilibrium tray efficiency eta(i)(eff). The methodology seeks to minimize an Error Objective Function that calculates the sum of the weighted and variance-normalized least-squares errors around the deethyleniser involving as many plant measurements as practical taking into account the wide range of accuracy inherent in the various measurements.