This paper uses analysis of the performance of the process as a means of supporting decision making for future investments in order to improve reliability and environmental issues in an existing cryogenic gas processing unit. An integrated model is presented that identifies opportunity areas for improvements in the operation of the unit. Dynamic simulation is discussed and the question of what is a required level of modelling detail in its applications is addressed. The methodology sets out how the problem can be targetted by considering relevant operators comments and analysing operational data. This can provide deep understanding of various interrelationships between multiple plant operating factors including maintenance activities, which could be essential for substantial improvements of process performance. The dynamic model validation should take into account records saved in the control system and look at how the model reproduces them. The model agrees in a satisfactory way with the actual performance of the unit, and it is used to evaluate the economic and environmental impacts that some of the possible failures could bring to the unit. Further work is planned to link the model to a more accurate model of the probability of failure occurrence, thereby supporting enhanced design of maintenance programmes.