Good Susheel, use pins DOUT/RDY, DIN, SCLK, CS of AD7799 to interface with 8051.
DOUT/RDY pin ofAD7799 to Rx pin of 8051
DIN pin of AD7799 to Tx pin of 8051
SCLK pin of AD7799 to CLK pin of 8051
CS pin of AD7799 to I/O pin of 8051.
Also refer AD7799 data sheet to write code & further information to communicate.