6pm is rush hour so you should take the bus from incheon airport into the city. it will take about an hour. once you come out of baggage claim, you should look for signs that say "airport bus." it is behind the information kiosk.

when you get outside, there will be a porter and "bus manager." tell them that you want to go to the renaissance hotel "Yeoksam-dong Gangnam-gu." they will put you on the right bus. they will also put the luggage on the bus for you and take it off at your arrival stop. no need to tip them.

some buses you have to buy the ticket in advance (you buy it from the bus manager before you get on the bus), and some buses you buy the ticket after you get off the bus. i took the bus this week from the city to incheon and it cost 14000 won (about $15 USD). not sure if the price is exactly the same, but it will cost about that much.

you should convert some money before you get on the bus. the buses do not accept credit card.

issy212 has good information. The airport bus is definitely the way to go. As I remember, the Renaissance is the next to last stop on the route.

The bus is definitely the best option. Take the blue KAL (Korean Airline) Limousine Bus - Line# 3. It costs 13,000 KRW. Also see the hotel's website: marriott.com/hotels/…