build_and_run.sh

mcs EnvPrinter.cs -out:EnvPrinter.exe
mcs Runner.cs  -out:Runner.exe
export MONO_GC_PARAMS=max-heap-size=1k 
mono Runner.exe mono EnvPrinter.exe MONO_GC_PARAMS

EnvPrinter.cs

using System;
using System.Collections.Generic;

namespace envvartest
{
    internal static class EnvPrinter
    {
        public static void Main(string[] args)
        {
            var varName = args[0];
            var res = Environment.GetEnvironmentVariable(varName);
            Console.WriteLine($"{varName} = {res}");
        }
    }
}

Runner.cs

using System;
using System.Diagnostics;
using System.Linq;

namespace runner
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            var cmd = args[0];
            string cmdArgs = string.Join(" ", args.Skip(1));
            ProcessStartInfo psi = new ProcessStartInfo(cmd, cmdArgs);
            psi.WindowStyle = ProcessWindowStyle.Hidden;
            var process = new Process {StartInfo = psi};
            psi.CreateNoWindow = true;
            psi.UseShellExecute = false;
            process.Start();
            int processid = process.Id;
            process.WaitForExit();
            int exitcode = process.ExitCode;
            process.Close();
            if (exitcode != 0) {
                throw new Exception("Exception during execution of external process: " + processid);
            }
        }
    }
}

Warning: In environment variable `MONO_GC_PARAMS': `max-heap-size` size must be a multiple of 4096. - Rounding up.
Warning: In environment variable `MONO_GC_PARAMS': `max-heap-size` must be at least 4 times as large as `nursery size`. - Setting to minimum.
Warning: In environment variable `MONO_GC_PARAMS': `max-heap-size` size must be a multiple of 4096. - Rounding up.
Warning: In environment variable `MONO_GC_PARAMS': `max-heap-size` must be at least 4 times as large as `nursery size`. - Setting to minimum.
MONO_GC_PARAMS = max-heap-size=1k

That basically means that export parameters for all internal Process are inherited.