Unless there is a physical defect with the fan/cooler/chip, or your case has inadequate ventilation, there was likely a mistake during reassembly.
Try this:
First, double check that the fan on your GPU is spinning, then remove the side panel from your case and check your temps again.
If there was a major improvident, you should add some more fans and check your cable management to increase airflow.
If not, you probably put to much thermal paste on your GPU when reassembling.
Try removing the cooler; thoroughly cleaning both the cooler and chip with high percentage isopropyl alcohol and a coffee filter; then reassembling using only a rice grained amount of TIM applied to the center of the chip.
If this fails, I would recommend contacting the manufacturer of your GPU and requesting a replacement.
Best not to mention that you have removed the cooler however; many manufacturers frown on that in the form of voided warranties...
Best luck!