What could be possible reasons for Atmega-chip and Xbee heating up?

If you're streaming data you're producing around three quarters of a Watt of power. Power = Amps * Voltage.

Add say, 20mA of microcontroller power to your 215mA...

235mA * 3.3V = 775.5 milliWatts

An Xbee Pro even at idle is going to raise temperatures a little running at 55mA, add your microcontroller and you're already at a quarter Watt.