The waste heat from a power plant (any kind), is not hot enough to make steam, unless you deliberately make it less efficient.

You can make hot water though.

Just a reminder that “steam” does not directly imply any specific temperature threshold as it is a function of relative pressure. Pumped into a (near) vacuum, warm water will turn to “steam.” (Cost of pulling a vacuum not considered.)

What's to say that demand needs to be met with waste heat? Couldn't a plant tap off and sell some of the steam that would otherwise be sent to the turbines?

Yes, and that has been done in the past. It comes at the cost of lower power production, though.

https://en.wikipedia.org/wiki/Bruce_Nuclear_Generating_Stati...

> It comes at the cost of lower power production

Lower electrical production, not lower power production.