The main goal of this thesis is to find out if a liquefied natural gas multistream heat exchanger numerical model is achievable. This should include several features usually neglected in nowadays available heat exchanger models, such as flow maldistribution, changes in fluid properties and heat exchanger dynamic behaviour. In order to accomplish that objective a simpler case is modelled. Efforts are put in achieving numerical stability.A counter flow natural gas and mixed refrigerant heat exchanger is modelled. Some important characteristics of the obtained model are: (1) it allows a dynamic study of the heat exchanger, (2) mass flow rate is a consequence of inlet and outlet pressure difference, (3) fluid properties change is taken into account, (4) it presents a time step control function and (5) fluid movement is not neglected. Some interesting numerical behaviours included in heat exchangers models design that have been observed during the course of this thesis are discussed. For instance, the comparison of the effects of choosing one heat transfers correlation or another.Dynamic response of the modelled heat exchanger during start up and during an abrupt change in mixed refrigerant inlet temperature are shown and discussed.