Well had you done any Google searches you would have come up with many different examples, but hey you go:
iptables -A PREROUTING -t nat -i ethX -p tcp --dport 80 -j DNAT --to 10.20.10.100:80
iptables -A INPUT -p tcp -m state --state NEW --dport 80 -i eth1 -j ACCEPT
of course change ethX to the NIC that is going to be accepting and forwarding
Then to make it a little more secure you could specify the source network so the 10.100.109.46/32 only go to the forwarded port:
iptables -A PREROUTING -t nat -i eth1 -p tcp --source 10.100.109.46/32 --dport 80 -j DNAT --to 10.20.10.100:80
some sites with some good examples:
Google can be your friend as there have been many before you that have wanted to do exactly what you are trying to do.