Since you propose to compile a kernel version which is different from the original, the process is fairly easily. When you compile the 2.4.18 kernel, you will have 2.4.18 in it's own directory, with it's own /lib/modules. Then copy /usr/src/linux-2.4.18/vmlinux (the compressed kernel image) to the /boot directory. Rename it: something like vmlinuz-2.4.18. This is the compressed kernel image you'll boot into. DON'T NAME IT VMLINUZ!. If you do, you'll overwrite the original which is in the boot directory.
This will give you two kernels to boot into, the original and the new addition. Then, edit grub.conf (or whatever RH uses) to add the new entry to the bootloader menu (just copy-paste the one that's there and edit to give it a new name on the menu, and to have it point to vmlinuz-2.4.18).