Enable forwarding and tell iptable to forward. Assuming you are working on port 80 then the following:
Enable port forwarding
# echo "1" > /proc/sys/net/ipv4/ip_forward
Tell iptable your forwarding rules
# iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination Y.Y.Y.Y:80