Only if the parts of the plant you plan to feed are edible by humans.
Potato (not sweet)
Never. The green above-ground parts are powerfully poisonous, as are the skin peelings if the potato has turned green. Non-green tubers:
Edible but pure carbs + little nutrition.
Pepper (Bell Capsicum annum)
Yep. Feed 'em. But I've never seen a discussion about iguanas eating other typs of peppers such as super-hot peppers like Jalepeno or Ghost Peppers.
You mean pepper LEAVES? I never heard. Force-feed some to your cat and see what happens. But plant poisons are not noxious to all species of vertebrates. Here in California I've watched tree squirrels sit and seriously eat amanita death caps.
Grape (vine leaves)
Tender new or mature tough leaves plus ripe (less acidic when ripe) fruit are very good. Wild grape foliage is OK too, as long as you're sure it's really a wild grape vine.
Do you have rocket, or roquet? I've read this one is good too. Also wild dandelions; roots plus plant plus blossoms.
Someone here has a list of relatively easy to get good British and Western Europe iguana munchies. Try the German iguana forums, as they are very technical about it all.