Dynamic simulation of helium plants using Aspen Hysys^®, V7.0 has been validated using cool-down temperature data from two existing helium plants under operation. Fluid properties of helium using 32-term MBWR EOS are found to be numerically unstable and Hepak^® data has been fed as a table. Metal properties in Aspen Hysys^® do not have temperature dependence and NIST data has been attached. Expansion engine and turbine characteristic equations also have been successfully incorporated into the process simulator. Unknown plant parameters have been estimated using Least Square Deviation method. The simulation results match closely with the real plant data after these customization and parameter estimations. A real time factor between 8 and 14 has been achieved.