The development of a relatively simple mechanistic model for an industrial ethylene cracking furnace is described, including the estimation of selected model parameters to improve model predictions. Energy balance equations are developed to account for radiative, conductive, and convective heat transfer in the radiant section, and for convection and conduction in the ultra-selective heat exchanger (USX) and in the transfer line exchanger (TLE). Kinetic schemes by Ranjan et al. and Sundaram and Froment are used to model the cracking reactions. The heat transfer model is combined with mass and momentum balances to model gas composition, pressure, and temperature changes as a function of position along the reactor tubes. Initial values and uncertainty ranges are assigned to 44 model parameters based on information in the literature and our industrial sponsor. A sensitivity-based technique and a mean-squared-error (MSE) criterion are used to select the appropriate subset of 22 parameters for tuning. Parameters are estimated and model predictions are validated using industrial data. Model predictions provide a good match to data that were not used for estimation.