BeerBorg

My kitchen stove will just barely bring and maintain 3.5 - 4 gallons of wort to a boil. So I have to put a top over my brew kettle but I leave it off to one side so that there is about 2" of an opening for the steam to escape.

I have heard some say that this is bad and will not allow the DMS to escape. But then I looked at this:
This is a picture of Anchor Brewing's brew kettles. They have lids on them with openings to let the steam escape.

Except for the scale I can see no difference in what I do and what they do. Am I wrong?

You are taking a risk. You dont know the details of those big kettles. They may have some way of forcing the steam out. While you will be having the steam drop back in your boil. If you dont ruin a batch go for it.

I have done many a boil with the lid 1/2 or 3/4th the way on due to wimpy heat source, and have had no DMS issues, so I'd guess you will be ok personally. YMMV with certain malt bills though - IE I have not tried this with an all Pilsner malt bill, that might be different - I just don't mash pilsner due to the extra time it requires and the quality of the bulk pilsner extract that I get from MoreBeer.

I know what you are saying man, and I feel your pain. My stove is the same way, can't do a 3.5-4 gal boil as it struggles to keep it going. So I had to switch to a propane burner once going all grain. If you are going to try the lid trick, the lid really helps to get up to a boil. That is the hardest part, you might only need the lid until about halfway through the boil, as the volume drops you can maybe remove the lid more and more. And I would try to do that last 15-20 minutes with the lid off and with as strong as a boil as possible to boil off any DMS that may be present. Your evaporation rate may be lower as well since your steam will condense and fall back in the kettle. What happens with those old brewery kettles I'm not sure but I think it's a little different.

Another option is finding one of these splatter screens. By design they wont trap the condensation but they keep enough heat in that it helps with the boil. I've used one and it does help. I would feel more comfortable using the lid just to get up to boil and switching over to this as soon as you can to keep a good boil going. But are you brewing all grain or with extracts and steeping grains? DMS isn't a concern much as the boil already took place with the extracts. It's all grain brewing especially with lighter malts like Pilsner where it's a concern. I mean there's many times extracts are barely boiled at all, like with late addition techniques.