Well, the simulations all panned out, but when I actually assembled the circuit, the op amp began operating like a switch. The output voltage is 3.4V until I reach about 4V input, and then it jumps to 10.3V. Is the LM324 the wrong op amp to be using here?
Update: nevermind, after checking and rechecking, I noticed I failed to jumper the feedback resistor to the op amp output. Now it works more like what I intended, but still does not ever turn off completely. I am assuming this is due to a small amount of leakage current across the transistor, despite being un-biased? Also, I've found that the best output obtainable on the LM324 using a 12V Vcc is about 10.3 Vdc, so I will be recalculating my resistors for that value.