MOSFET Gate Driver Simulation Problem
The IR2112 uses a charge pump (C2 and D1) to increase Gate drive voltage. For this to work properly you must apply PWM to HIN, which keeps C2 charged up.

Each time the FET turns on it pulls the load voltage high. Since C2 is already charged up to nearly 15V, and D1 stops it from discharging into the 15V supply, the boost voltage at VB will go up to almost 15V above the FET's Drain voltage (ie. to about 115V). The driver then switches between this boost voltage and VS, providing close to 15V Gate drive to the FET even though it is way above VC. C2 will loose some charge through the gate drive circuit, but when the FET is turned off again (during PWM off time) it is recharged through the load (R4).  

If you hold HIN high continuously then C2 will continue to discharge through R1 and R2. Eventually the drive voltage will drop below the FETs threshold voltage and it will start to turn off, then boost voltage will drop and you will be left with only the 15V from VS. The load voltage will drop even lower because the IRF840 needs about 6V to turn on. 

Another potential problem with your circuit is that C2 is being discharged through R1 and R2 almost as fast as it is being charged through R4. Over many PWM cycles the average voltage on C2 will drop, reducing Gate drive. At 50% PWM or above the drive voltage might drop below the FETs threshold voltage, causing it to turn off too soon. This can be fixed by either increasing the value of R2 or reducing the value of R4.