I am renovating a condo in a pre-war apartment building in Chelsea in Manhattan, NYC. I want to replace the ugly ancient cast iron radiators with modern flat wall mounted units. Does anyone have experience with this. Thanks.

That's a completely different type of heating system so you can get a plumber to come in and cap the radiator heating units and remove the cast iron and then install the wall mounts accordingly. Depending on the construction of your unit (brick or granite exteriors), it might not be plausible. 

I am bit unclear on you response. Here is a picture of the current radiator that I want to replace. 

What is it that is capped? Wouldn't the new flat unit simply attached to the pipe coming up through the floor (probably with an adaptor)?

You need to close out the gas line and plumbing going to the radiator that hears the cast iron as you will not use it. That is called capping the unit and effectively disabling it from use since you will be using the wall mounted (aka mini split) units for the property. 

Originally posted by @Lien Vuong :

You need to close out the gas line and plumbing going to the radiator that hears the cast iron as you will not use it. That is called capping the unit and effectively disabling it from use since you will be using the wall mounted (aka mini split) units for the property. 

Yes, but the current radiator is a steam radiator. There is no gas line involved. Just an old fashioned clanking steam radiator. You can see the little whistling thingy attached at upper right of the radiator.