What's the normal operating temperature for a unibody plastic Macbook?
It's the fans. They are clogged with dust and other debris from the years of use. Notebooks are not air-tight and intake quite a bit of air over their lifetime (they need to push the air to the central components like the CPU/GPU to cool them). The units must be opened and cleaned periodically (depending on environment) to maintain peak operating performance. High fan speeds and poor thermal dissipation are signs that the fans aren't doing their job (they are likely clogged with dust and the exhaust is also likely blocked, preventing the output of the hot air blown off from the processing units). Computer fans work by pushing air taking from the environment onto the CPU/GPU. The hot air blown off the chips move through air vents in the back of the unit. Over time, dust settles on the fans, dropping their efficacy, and the vents, preventing the hot air from escaping. This bakes the components and results in the spike in over temperature.

Visit www.ifixit.com and pull up a guide on how to partially dismantle your notebook. If you are not technically proficient, bring it to a credible repair shop.

There are no "normal" operating temperatures for Apple notebooks, but there is an acceptable gradient. 82 degree Celsius is pushing the limits. The CPU's thermal threshold is 105 degrees Celsius (when that temperature is reached, the system will shutdown to prevent thermal damage).

And just an aside, regardless of locale, all temperatures should be reported as Celsius, never Fahrenheit when dealing with computer components. Official documentation always refers to Celsius.