Hardware-in-the-loop systems are a type of Hard real-time system that require a faster than real-time simulation. A method of running hard real-time systems with slower than real-time simulations is devised. Instead of waiting for the real-time subsystem output to a simulation, multiple simulation input predictions are generated and used to run several parallel simulation paths independent of the real-time subsystem. The simulation path results are stored until needed by the real-time subsystem at which time one is selected and returned to it to continue the execution of the system. A test case using a small modular reactor nuclear power plant hardware-in-the-loop system was used to demonstrate the effects of slower than real-time simulations on a hard real-time system and to show that this method allows such a simulation to be used.