The Phillips 66 Sweeney Refinery and Petrochemical Complex is a highly integrated complex consistingof refining, chemicals, and ethylene units. The Complex received a Texas Natural Resource Conservation Commission (TNRCC) Flexible Permit to install predictive emissions monitors (PEMS) at eight furnaces of Ethylene Unit 22. Unit 22 produces approximately 1.8 million pounds per day of ethylene. Hydrogen in the fuel gas fluctuates between 20% and 80% depending on economics and the operational status of upstream and downstream units. Maintaining consistent fuel gas hydrogen levels and unit upsets made it difficult to train the models. The PEMS is certified and has been in full operation since March 1996. The difficultes and successes of the PEMS certification process is discussed in this paper.