Here are my suggestions:
1. Do calibration and correct the Offset error. Go through this App Note on how to do accurate measurements using ADC. - http://www.cypress.com/?rID=40988
2. Use the ADC range as 0-6*Vref and keep the VRef to internal 1.024V itself
Why do you need 10Ksps samples to be taken from a Temperature sensor?
If the input Buffer of a Delta Sigma ADC is enabled, there is a small yet inevitable amount of Offset Error you'll see. This is an inherent behaviour of any op-amp based Buffer which comes along with the Higher Input impedance of Buffer.
Did you try measuring the output of LM35 after bypassing the Buffer in ADC? If the output impedance of LM35 is small enough, you will see that the resulting output in this configuration has no Offset Error. The Small amount of Gain Error which the Buffer would have introduced will also be eliminated.
Let us know if this helps.