During the design, commissioning and operation of an FEL it is important to have a good understanding of the implications of design choices on beam figures of merit. This simulation engine combines a full state-space model of the RF plant (including cryomodules and electro-mechanical interactions), LLRF controller as well as a characterization of beam properties such as energy, bunch length, arrival time, etc. as it propagates throughout the Linac. The combination of these models with the ability to introduce both correlated and uncorrelated noise sources at any point of the machine, allows for a complete transposition of noise sources to beam performance parameters, including frequency dependence.