Running 150 degrees F how do I cool it down?

150 degrees is nothing. I've heard of it reaching the boiling point of water at sea level. Those applications that measure the temperature are unreliable. What they might give is a clue if your machine is showing some other signs of misbehavior that coincides with temperature spikes. Apple's official specs only discuss outside environmental conditions, not inside of the machine. Such signs could be kernel panics* (four language screens telling you to restart, which may also be caused by other issues), automatic power offs while working with it and not hitting any key or touching any cables or having Energy Saver settings to force it off.

What exactly is running at 150 degrees F?

If it's the Power Supply or the GPU diode, both of these, usually, run on the hotter side .

If this is heat coming from the CPU, hard drive or optical drive, then there could be a problem.

Despite what some other experienced users here will tell you 150 degrees F is hot enough to be of concern.

When was the last time any of the iMacs vents, slots and ports vacuumed out from dust, lint and dirt?

You need to power down your iMac, disconnect everything, even the power cable, lay the iMac, screen down, on a soft towel or blanket before starting the cleaning.

When cleaning away surface dirt from all of the openings around your iMac, use a vacuum with hose and crevice tool ( no brush attachments as these have the potential to cause a static discharge). Also, clear away any dust, lint and dirt from the RAM slot by unscrewing the access cover and slowly and lightly vacuum inside that area.

Once this is complete, reconnect everything on your iMac and start it up.

report back with your iMac's internal temps.

These are normal temps, shown by Temp Monitor.

Try doing an SMC reset on the mac, this affects the fans and other internal items on a mac. To do this on an imac you simply need to power down and then remove the power cord from out of the back of the imac. leave it out for 15 seconds. plug it back in wait another 5 seconds then turn it on see if the fan and temperature adjust any