I have a natural gas dryer that I want to use in a location that only has propane gas. I know it can be converted, but I've been told that propane burns a lot cooler than natural gas and it takes clothes much longer to dry. Is this true?

.... Is this true?
There could be a slight difference in the flame temperatures but I'm sure you wouldn't notice that difference. I think you might mean the "heat content". Natural gas has a heating value of about 1000 BTU per (gaseous) cubic foot while propane has a heating value of 2500 BTU (gaseous) per cubic foot. There are some technical definitions that apply here but, trying to simplify it, a BTU is the amount of heat energy required to raise the temperature of 1 pound of water 1 degree Fahrenheit. Then think of a rubber balloon with one cubic foot capacity that has been filled with nat gas and you have 1000 BTU's of energy available to raise the temp of one hundred pounds of water ten degrees Fahrenheit. Using this same analogy, fill the same balloon with propane gas and you could heat one hundred pounds of water twenty five degrees. I've really made a simplistic explanation here, and not precisely correct in the calculations as there's items like efficiency, heat loss, etc, but you can get the big picture.

I.E. : propane produces more heat per equal quantity than natural gas.
Therefore make sure that the burners are modified for using propane gas.
That done there hardly will be any difference in drying capacity between the two!

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Link to the source of this information! (http://en.allexperts.com/q/Energy-Industry-Oil-2441/2008/3/natural-gas-vs-liquid.htm)